I am currently using Avast! Free Antivirus 5.0.594and Comodo FW 4.1.150349.920 on my PC without any conflicts or slowdowns in boot-time, regular performance or network speed. Avast! Free Antivirus comes with a ton of features that are not even available in commercial anti-virus products.

File system shield:
This shield scans files that are currently in use (active files)Mail Shield:
I use ThunderBird and Avast gave me a warning to disable SSL/TLS security in Thunderbird. After I disabled this in ThunderBird, Avast was able to scan all my mails for infections.

Whenever a new mail is received, Avast Mail shield will automatically check it for infections
Webshield:
This shield protects your PC from infections/scripts from the webpages you visit. (Does not slow your browsing speed)

Network Shield:
This shield protects your PC from potentially dangerous websites. It will automatically block your browser from loading such websites. An automated voice message alert will be played after a malicious url/threat has been blocked.

I tried accessing a proxy website, Avast blocked it automatically and prevented FireFox from loading the site.

Comparison of Avast free and commercial versions, surely the free version is outstanding and provides perfectly good security even for a new user.

Avast has a couple of special scans available if you enable them in Settings>Basic Settings>Special scans

Here is a screenshot showing Avast with the scans available by default

Now to open Avast’s Settings

Enable the Special scans by ticking the checkbox as shown in the screenshot below

Now the New scans are available as shown below

Important Tip to make to speed up Avast scans :
To make the Full System Scan even faster, configure it to actually take advantage of the persistent cache. To do this, open the Full System Scan details, click the Settings button and check the box â€œSpeed up scanning by using the persistent cacheâ€ on the Performance page.

License registration:
Avast! Free Antivirus requires free registration after 30 days of usage.
Avast can be registered in two ways, 1. From their website and 2. through the Avast interface

Registration through the interface is easier and faster.
You need to provide your name, email id, choose your age group and answer an optional question to register for a free 1 year license.

Summary:
I find Avast! Free Antivirus to be very light on resources, scans are blazing fast and reliable.
Besides Avast and Comodo, I also use SuperAntispyware, Malwarebytes antimalware, Winpatrol and Spybot to keep my PC free from infections. (All four have free versions available)

I recommend you to try Avast! Free Antivirus before buying any commercial AV.